Compounds represented by the following general formula (1a): ##STR2## wherein B.sup.1 represents a nucleic acid base, exhibit an antiviral activity. Particularly, they exhibit a strong activity against herpes simplex virus 1 and 2 (HSV-1,2), human cytomegalovirus (HCMV), hepatitis B virus (HBV), human immunodeficiency virus (HIV), etc. Further, there are disclosed a variety of analogues of these compounds (EP 0335355-A2, EP 0358154-A2, EP 0366059-A2).